The main issue is one of making certain that DSM of Elvenkind and DSM of Speed are not too common. Craftsmanship is just a hack around the fact that it's hard to control overall ego item rarity on a specific item type (thus, there will be lots of ego DSMs because most items are egos after awhile). The Craftsmanship ego is comparatively vastly more common just to make sure that the majority of ego DSM are not significantly more powerful than non-ego DSM.

Seems reasonable that it would give a little extra stealth if it's made better and a little quieter when moving in it.

Or just get rid of regular dragon scale mail. The idea of crappy discount dragon scale mail is silly to me.

You can do this more naturally if you make armour weight count against stealth or matter more to mages. Then stuff like mithril being lighter is useful straight up through it's weight. How it is now past a certain pt weight stops mattering that much.